the marines original mission and the one that they'll return to after this war is amphibious assault. the army's is a land. they can both do each others job, but they specialize in separate types of combat.
the marine corps is also significantly smaller, they are under the navy in terms of funding, so they're given older weapons and ****** living quarters which just helps to piss them off and make them meaner.
also, every marine goes through additional combat training after boot camp, something that no other branch does.